The use of formaldehyde based resins has been fundamental in the manufacture of many wood and textile products for many years. In the wood products industry urea-formaldehyde (UF) resin has been in prevalent use as an adhesive in particle board manufacture with phenolic resins and isocyanate used to a lesser extent. Low cost and ease of use of UF resins have been the driving forces for their acceptance. Resins used in the textile industry are a different type than those used in wood products and have generally been used as chemical finishing agents to impart durable press or shrink resistant properties to cellulosic based fabrics such as shirting and knit materials. In the resin finishing of cellulosic fiber fabrics, nitrogen containing methylol compounds have been preferred because of their ease of application and low cost. The resin most extensively used in recent years for durable press finishes has been dimethyloldihydroxyethyleneurea (DMDHEU) or a modified version thereof.
Associated with products that contain UF and DMDHEU resins is the release of formaldehyde. Such is measured by large chamber test method FTM-2 for wood products and by AATCC Test Method 112-1984 for textile products.
Formaldehyde was first prepared by A. M. Butlerov in 1859 as a by-product of the synthesis of methylene glycol. It was known to have a pungent odor and to be extremely irritating to the mucous membranes of the eyes, nose, and throat. Because it was a severe irritant even in small amounts, research and development over the years was devoted to developing resins, particularly in the textile industry in order to minimize this form of toxicity. Within the last few years formaldehyde has come under scrutiny as a potential carcinogen. This has served to increase pressure by governmental agencies to reduce or eliminate formaldehyde evolution from wood and textile products.
The chemical industry has responded to the concern over formaldehyde by modifying some of the present formaldehyde based resin systems and by developing non-formaldehyde resins. Unfortunately, the non-formaldehyde resin systems have been costly to produce. Modified formaldehyde based resins have proven more successful in reducing the level of released formaldehyde from wood and textile products. For example, UF resins used in particle board manufacture have been modified by changing the mole ratio of formaldehyde to urea in the resin system. High fuming resins with mole ratios of 1.5 to 1 have been modified to low fuming resins of 1.1 to 1 mole ratios. This has resulted in a significant reduction in released formaldehyde from particle board. In the durable press finishing of fabrics the pendant N-methylol group (N--CH.sub.2 --OH) in DMDHEU has been restructured by "capping" with a polyol to form N--CH.sub.2 --OR where R is traditionally a methoxy of a diethylene glycol ether group. This chemical restructuring stabilizes the resin molecule to hydrolysis thereby reducing the liberation of free formaldehyde from fabrics treated with such modified resins.
In addition to the foregoing there are methods described in the literature other than resin modification that have had some success at reducing the levels of released formaldehyde. Most of these methods have involved additional production processes such as after-washing or post spraying. These are generally considered costly, although effective, alternatives. A method more acceptable to manufacturing has been the use of additive chemicals called formaldehyde scavengers that can suppress or reduce the liberation of free formaldehyde from wood or textile products containing formaldehyde based resins.
Development of formaldehyde scavengers has been more prevalent in textiles than in wood products and dates back some twenty years. Generally, in textiles a formaldehyde scavenger is added to a resin mixture. The mixture is applied to cellulosic based materials in manufacturing processes to produce a product characterized by a reduced level of residual formaldehyde over that of non-treated products. Articles that discuss formaldehyde scavenger development in textile durable press finish of fabrics include R. S. Perry et al, Textile Chemist and Colorist, Vol. 12, No. 12, December 1980, p. 311, Northern Piedmont Section, AATCC, Textile Chemist and Colorist, Vol. 13, No. 1, January 1981, p. 17; C. Tomasino and M. B. Taylor II, Textile Chemist and Colorist, Vol. 16, No. 12, December 1984, p. 259; and Perry, R. S., U.S. Pat. No. 4,127,382. Some of the compounds discussed in these articles have indeed been found to be effective as formaldehyde scavengers per se. For example, urea and other urea compounds such as ethylene urea and carbohydrazide significantly reduce formaldehyde release from fabrics but suffer themselves from odor formation, fabric discoloration, chlorine retention and buffering of the cure. Nitrogen containing aromatic hetercyclics such as pyrrole, indole and triazoles are also effective scavengers but tend to yellow fabrics at the application levels needed to reduce formaldehyde release to current standards. Non-aromatic alcohols have proven to be effective as formaldehyde scavengers without imparting detrimental physical properties to the fabric. Sorbitol, methoxy glucoside and diethylene glycol are scavengers presently being used to reduce formaldehyde levels.
Although alcohols when used as an additive scavenger can reduce formaldehyde levels of, for example, DMDHEU, their effectiveness depends on the form of the resin. For example, it is not unreasonable for DEG or sorbitol at 3 to 4% on bath weight to reduce the level of a DMDHEU resin which is 800 ppm odor formaldehyde on control fabric to 300 ppm on test fabric for a 63% formaldehyde reduction. Fabric finished with a modified DMDHEU resin may only have 100 ppm odor formaldehyde on the control fabric. The addition of polyols in the durable press finishing formulation as a scavenger may not reduce the formaldehyde level in the test fabric to any significant extent, at least not at reasonable usage levels. In this case polyols can be ineffective as additive formaldehyde scavengers.
It thus is seen that at present there is not a commercially viable formaldehyde scavenger available to reduce odor formaldehyde of a DMDHEU or modified DMDHEU resin as measured by AATCC test method 112-1984 to 25 ppm or less. Chemicals that have been reported to be able to reduce formaldehyde levels on fabrics to very low levels are carbohydrazide and dimethyl-1, 3-acetonedicarboxylate. However, the former adversely affects fabric properties and the latter is water insoluble and costly.
In wood products, particularly particle board, urea has been the scavenger that has received the greatest attention. Other chemical systems such as resorcinol, peroxides, and ammonia treatment have proven marginal in results and expensive. The usage of urea does not pose problems with particle board as it does in textile fabrics, however it must be carefully used or it will adversely affect physical properties of the particle board itself.
In general, urea is not added to the resin mix but is sprayed in a 40% solution as a separate application on the wood particles after resin application. The application level is approximately 0.3% based on the total board weight. This is generally the upper limit of usage on low fuming resins before board properties are affected. At present, the effectiveness of urea as a formaldehyde scavenger in particle board, as well as in other composition board products, is rather variable depending on manufacturing conditions and resin type. Only the low cost of urea accounts for its continued usage. As long as the wood products industry uses UF resins, and as long as formaldehyde reduction remains a governmental priority, there will be a need to develop a more effective scavenger. It is to the provision of such therefore, that the present invention is primarily directed.